Star Evolution - how it works

People have long been interested in the causes of the burning of stars in the sky, but we really began to understand these processes from the first half of the 20th century. In this article I tried to describe all the main processes occurring during the life cycle of a star.

The birth of stars

Star formation begins with a molecular cloud (which includes 1% of the total interstellar substance by mass) - they differ from the usual gas-dust clouds for the interstellar medium in that they have a higher density, and a much lower temperature - so that atoms can begin to form molecules (mostly - H²). This property itself does not really matter, but the increased density of this substance is of great importance - it depends on whether a protostar can form at all and how long it will take.
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These clouds themselves, at a low relative density, due to their huge size can have significant masses - up to 10 ⁶ Solar masses. Newborn stars who did not have time to throw away the remnants of their “cradle” warm them up, which for such large clusters looks very “impressive” and is a source of wonderful astronomical photographs:



"Pillars of Creation" and a video about this photo of the Hubble telescope:



The Omega Nebula (part of the stars - is the “background”, the gas glows due to the heating of the stars by radiation):


The very process of discarding the remnants of the molecular cloud due to the so-called "solar wind" is a stream of charged particles that are accelerated by the electromagnetic radiation of a star. The sun loses due to this process a million tons of matter per second, which for him (weighing 1.98855 ± 0.00025 * 10 ²⁷ tons) is nothing. The particles themselves have a huge temperature (about a million degrees) and speed (about 400 km / s and 750 km / s for two different components):



However, the low density of this substance means that they can not cause much harm.

When gravitational forces begin to act, the compression of the gas causes a strong heating, due to which thermonuclear reactions begin. The same warm-up effect of the colliding substance served as the basis for the first direct observation of an exoplanet in 2004:


Planet 2M1207 b at a distance of 170 of St. years from us.

However, the difference between small stars and gas giant giants is precisely that their masses are not enough to sustain the initial thermonuclear reaction, which in general consists in the formation of helium from hydrogen — in the presence of catalysts (the so-called CNO cycle — it is for stars of II and I generation, which will be discussed below):



We are talking about a self-sustaining reaction, and not just the presence of its fact - because at least the energy for this reaction (and therefore the temperature) is strictly limited from below, but the energy of motion of individual particles in a gas is determined by the Maxwell distribution:



And therefore, even if the average gas temperature is 10 times lower than the “lower limit” of a thermonuclear reaction, there will always be “cunning” particles that will collect energy from their neighbors and gain enough for a single case. The higher the average temperature, the more particles the barrier can overcome, and the more energy is released during these reactions. Therefore, the generally accepted boundary between the planet and the star is the threshold at which the thermonuclear reaction not only takes place, but also allows you to maintain the internal temperature in spite of the radiation of energy from its surface.

Stellar population

Before talking about the classification of stars, it is necessary to retreat, and return 13 billion years ago - at the time when the first stars began to appear after the recombination of the substance. This moment would seem strange to us - after all, we would not see any stars other than blue giants at that moment. The reason for this is the absence of "metals" in the early Universe (and in astronomy, all substances are called "heavier" than helium. Their absence meant that the ignition of the first stars required a much larger mass (within 20-130 solar masses) - because without the “metals” the CNO cycle is not possible, and instead there is only a direct hydrogen + hydrogen = helium cycle. This should have been the star population of III (due to their enormous weight, and their early appearance - in the visible part of the Universe they no longer remained).

Population II is the stars formed from the remnants of stars of the III population, they are more than 10 billion years old, and already contain in their composition "metals". Therefore, once in this moment, we would not have noticed any peculiar oddities - among the stars there were already giants and “middling” - like our star, and even red dwarfs.

Population I - these stars are already formed from the second generation of supernova remnants, containing even more "metals" - these include most modern stars, and our Sun - as well.

Star classification



The modern classification of stars (Harvard) is very simple - it is based on the separation of stars according to their colors. In small stars, the reactions go much slower, and this disproportion causes a difference in the surface temperature, the greater the mass of the star - the more intense the radiation from its surface:


Distributions of colors, depending on temperature (in degrees Kelvin)

As can be seen from the graph of the Maxwell distribution above, the reaction rates do not grow linearly depending on temperature — when the temperature approaches the “critical point” very close, the reactions begin to go ten times faster. Therefore, the life of large stars can be quite short on an astronomical scale - just a couple of million years, this is nothing compared to the estimated lifetime of red dwarfs - a whole trillion years (for obvious reasons, not a single star has yet gone out, and in this case we we can only rely on calculations, but their lifespan is clearly more than a hundred billion years).

Star life

The life of most stars proceeds on the main sequence, which is a curved line extending from the top-left to the bottom-right corner:


Hertzsprung - Russell Chart

This process may seem rather dull: hydrogen turns into helium, and this process continues for millions and even billions of years. But in fact, on the Sun (and other stars), even during this process, something happens on the surface (and inside) all the time:


A 5-year video taken from NASA Solar Dynamics Observatory photographs launched as part of the Life with a Star program, shows the view of the Sun in the visible, ultraviolet, and X-ray light spectra.

The complete process of thermonuclear reactions in heavy stars looks like this: hydrogen - helium - beryllium and carbon, and then several parallel processes begin to take place, ending with the formation of iron:



This is due to the fact that iron has a minimum binding energy (calculated per nucleon), and further reactions proceed already with absorption, and not release of energy. A star all his long life is in balance between the forces of gravity, compressing it, and thermonuclear reactions, which radiate energy and tend to "push" the substance.

The transition from the burning of one substance to another occurs with an increase in temperature in the core of the star (since each subsequent reaction requires an ever-increasing temperature — sometimes by orders of magnitude). But despite the rise in temperature - in general, the “balance of power” persists until the very last moment ...

End of existence

The processes occurring at the same time can be divided into four variants of the development of events:

1) The mass depends not only on the duration of the star’s life, but also on how it will end. For the “smallest” stars - brown dwarfs (class M), it will be completed after the burning of hydrogen. But the fact that heat transfer in them is carried out exclusively by convection (mixing) means that the star makes the most efficient use of its entire stock. And also - it will be very careful to spend it for billions of years. But after consuming all of the hydrogen, the star slowly cools, and will be in a state of a solid ball (like Pluto) consisting almost entirely of helium.

2) Next come the heavier stars (to which our Sun also belongs) - the mass of this possible future star is bounded above by 1.39 solar masses for the remainder formed after the red giant stage (Chandrasekhar limit). The star has enough weight to ignite the formation of carbon from helium (of course, the most common nuclides are helium-4 and carbon-12). But the reactions of hydrogen-helium do not cease to go - just the area of ​​their flow into the outer layers of the star, still saturated with hydrogen. The presence of two layers in which thermonuclear reactions occur leads to a significant increase in luminosity, which causes the star to “swell” in size.

Many people mistakenly believe that until the moment of the red giant, the luminosity of the Sun (and other similar stars) gradually decreases, and then begins to grow sharply, in fact, the luminosity increases in the entire main part of the star’s life:



And on the basis of this, wrong theories are being built, that in the long run - Venus is the best option for human settlement - in fact, by the time we have technologies for terraforming modern Venus, they may turn out to be hopelessly outdated and simply useless. . Moreover, according to modern data, the Earth has high chances to survive the state of the “red giant” of the Sun, on its border, but Venus has no chance, and “everything that has been acquired by overworking labor” will become part of the “replenished” Sun.



At the red giant stage, a star not only significantly increases the luminosity, but also begins to rapidly lose mass, due to these processes, the fuel reserves run out quickly (this stage is at least 10 times less than the hydrogen burning stage). After which the star is reduced in size, turns into a white dwarf and gradually cools.

3) When the mass is above the first limit, the masses of such stars are enough to ignite subsequent reactions, up to the formation of iron, these processes ultimately lead to a supernova explosion.



Iron practically does not participate in thermonuclear reactions (and certainly does not release energy), and simply gathers in the center of the nucleus until the pressure exerted on it from the outside (and the gravitational force of the core itself from the inside) reaches a critical point. At this moment, the force compressing the core of the star becomes so strong that the pressure of electromagnetic radiation is no longer able to keep matter from compressing. Electrons are "pressed" into the atomic nucleus, and are neutralized with protons, so that practically only neutrons remain inside the nucleus.

This moment has a quantum basis, and has a very clear boundary, and the composition of the core consists of fairly pure iron, so the process is catastrophically fast. It is assumed that this process occurs in seconds, and the volume of the core drops 100,000 times (and its density increases accordingly):



The superficial layers of the star, being without support from below, rush inward, falling on the formed "ball" of neutrons, the substance bounces back, and an explosion occurs. The blast waves rolling through the thickness of the star create such a compaction and an increase in the temperature of the substance that reactions begin to form with the formation of heavy elements (up to uranium).

These processes are based on neutron capture (r-process and s-process) or proton capture (p-process and rp-process), with each such chemical element increasing its atomic number. But in a normal situation, such particles do not have time to "catch" another neutron / proton, and decays. In the same processes occurring inside a supernova, reactions proceed so quickly that the atoms have time to "slip" most of the periodic table, and not broken up.

Thus, the formation of a neutron star:


4) When the star’s mass exceeds the second one, the Oppenheimer – Volkov limit (1.5–3 sun masses for the remainder or 25–30 masses for the original star), in the supernova explosion there is too much mass of matter, and the pressure is not able to restrain even quantum forces.

In this case, there is a limit due to the Pauli principle, which says that two particles (in this case, we are talking about neutrons) cannot be in the same quantum state (this is the basis of the structure of an atom consisting of electron shells, the number of which gradually grows with atomic number).

The pressure squeezes the neutrons, and the further process becomes irreversible - all matter is squeezed into one point, and a black hole is formed. It itself does not affect the environment in any way (with the exception of gravity of course), and can shine only due to the accretion (simply falling) of the substance on it:



As can be seen from the sum of all these processes - the stars are a real storehouse of physical laws. And in some areas (neutron stars and black holes), these are real physical laboratories with extreme energies and states of matter.

References:

Review article on galspace

Postnauka - Neutron stars and black holes (video series):